1. Field of the Invention
The present invention relates to the field of refrigeration and more particularly, to refrigeration systems incorporating cold storage that are capable of essentially unpowered operation for substantial periods of time.
2. Prior Art
Heretofore various types of refrigeration/cooling apparatus have been disclosed that incorporate some form of cold storage. In some cases the cold storage is used to supplement the cooling capacity of the apparatus during peak load conditions, in other cases it is used to allow unpowered operation of the system at times when external power is expensive or unavailable.
These systems vary in their structural arrangement, the type of cold storage device used, the method used to transfer heat to and from the cold storage device, and in their cooling capacity: i.e. whether they can be utilized for air conditioning, refrigeration, or freezing.
U.S. Pat. No. 4,129,014 discloses a "Refrigeration Storage and Cooling Tank" in which the cold storage device consists of a gas tight housing containing a "heat pipe liquid" enclosing a number of containers of a freezable liquid. The gas tight housing contains two separate heat exchanger pipe assemblies: one is connected to a conventional refrigeration circuit and is used to "charge" the cold-storage device; the second is connected to an outside heat exchanger and contains brine. The brine is circulated to cool the outside heat exchanger. This system therefore uses three heat transfer fluids: the refrigerant in the refrigeration circuit, the "heatpipe liquid" in the gas tight housing, and the brine flowing through the outside heat exchanger.
U.S. Pat. No. 2,664,716 discloses a refrigeration system in which small containers containing a "congealable" eutectic liquid are used as cold storage devices. The containers are manually placed in a conventional freezer and frozen when excess cooling capacity is available. The stored cold is used to supplement the cooling capacity of the freezer at times of heavy cooling loads, as when large quantities of food must be rapidly frozen. The containers must be manually removed when they become discharged, or when the space they occupy is needed to store more frozen food.
U.S. Pat. No. 2,512,576 discloses a refrigeration system in which a tank of water is used as the cold storage device. During times of low cooling loads, refrigerant from a conventional refrigeration system is circulated through coils immersed in the water in the tank, cooling and freezing the water. At times of high cooling loads the ice is used to cool the refrigerant before it is circulated in a refrigerator or cooler. This system is not capable of non-powered cooling.
U.S. Pat. No. 2,246,401 discloses an air-conditioning system that also uses a tank of water as the cold storage device. The system comprises two refrigeration circuits: One is used to freeze the water in the tank at times of light cooling loads, the other to cool air, either directly or by first cooling water. In times of heavy cooling loads, two methods are disclosed of utilizing the cooling capacity of the ice in the tank. According to the first method, water from the melting ice is mixed with water cooled by the refrigeration machine and circulated through a heat exchanger over which air is blown. According to the second method, a portion of the refrigerant evaporated by the air being cooled, instead of passing through the compressor and condenser, passes through the water tank and is condensed by the melting ice. This system therefore requires two refrigeration circuits, and the stored refrigeration is used only to augment the cooling produced by a conventional refrigeration system.
U.S. Pat. No. 1,957,313 discloses a refrigeration system in which the cold storage device consists of an envelope of "cryohydrate composition" surrounding the refrigerated space. A refrigerant line is in thermal contact with both the refrigerated space and the cryohydrate composition. A conventional refrigeration machine is operated intermittently to freeze the cryohydrate. Once the cryohydrate is frozen, the refrigeration machine is turned off. Heat is transferred by conduction from the refrigerated space to the cryohydrate, causing it to melt. In this system, the cold storage device must be in direct physical contact with the refrigerated space.
U.S. Pat. No. 1,891,714 discloses a refrigerating system for use as an air conditioner in which water is used both as the cold storage medium and as a heat transfer fluid. A conventional refrigerating machine is used to cool water in a tank. The cooled water is then used to cool air. At times when there is only a small air-conditioning load, only part of the cooling capacity of the refrigerating machine is absorbed by the air. The water decreases in temperature and eventually starts to freeze. When the air-conditioning load is high, the water temperature rises, melting the ice. The heat absorbed by the melting ice supplements the cooling capacity of the refrigerating machine. This system therefore comprises two heat transfer circuits: one, a conventional refrigeration circuit used to cool the water, and two, a water circuit used to cool the air.
U.S. Pat. No. 3,744,264 discloses a refrigeration system that, while not incorporating a cold storage device, is capable of limited non-powered cooling. This system is comprised primarily of a conventional refrigeration system including a compressor, a condenser, expansion valve, and an evaporator. Both the condenser and the evaporator are shell and tube heat exchangers. Water to be cooled is circulated through the evaporator, cooling water is used to cool the condenser. The system is capable of non-powered cooling only when the condenser water is cooler than the evaporator water. In that case, a single separate refrigerant line is used to by-pass the compressor and form a direct link between the evaporator and the condenser. Refrigerant vaporized in the evaporator flows through the bypass line to the condenser, where if the condenser water is cool enough, it condenses. The refrigerant flows back to the evaporator. Since this system features only a single refrigerant line for non-powered cooling, its non-powered cooling capacity is extremely limited.
The cooling coils of a refrigeration system that is operated at temperatures below the freezing point, if exposed to moist air, generally require periodic defrosting to prevent excessive build-up of ice. In conventional defrosting systems, such as disclosed in U.S. Pat. Nos. 3,638,444 and 3,677,025, hot, compressed, refrigerant from the compressor is by-passed around the condenser and expansion valve directly to the evaporator, where it melts any accumulated frost.